You can run multiple Mendix apps on one server. However, you will need a license per app.
This only applies to on premises situations. In the cloud you won't have to worry about servers or hosts.
I think you can. Recently working for a company which uses one Mx licence for 3 labels now. It totally depends how you set up your IT landscape with of coure respecting the license constraints as max concurrent users etc.
An example would be that each app is pointing to one url (deeplink ) where each app has his own own modules (domain models). In fact 1 mendix app is also a collection of micro apps (modules).
1 - you have to set up your domain model and pages according to those 3 "apps" under the 1 app. Read it as brings more complexity and need for experienced developer.
2 - one accp and one production environment, should not be a problem if you can handle it
3 - all users will be seen under the one license
I think nowadays there is not such thing as one application, each application is data (pages & logic) with server constraints. When you respect that in your IT architectture you can also build Mx main app that consists of 10 multi lable apps, but it will bring complexity with it.
Ideal situation would -of course- be that each app has his own (T)AP...but sometimes it is better and less work to have all the stuff in one Mx license/main app...I did it more times. If you need help you can DM me.